Not following a clear financial plan.

"Without a written financial plan, there's no destination in mind, and there's no tangible concept of how much is needed to be saved," says Mark Avallone, a certified financial planner and president at Potomac Wealth Advisors. "But if you have structure in your monthly budget, you know what you can and cannot afford."

A clear financial plan includes savings goals. Start by establishing what is important to you, and get an idea how much and for how long you would have to save in order to reach your goals. Next, start putting away money.

Buying things you can't afford.

Trying to outshine your neighbors or coworkers can be tempting, but it can also be detrimental to your finances. If you can't afford it, you shouldn't buy it.

"You have to monitor your spending and limit aspirational purchases," says Avallone. "Just because you see other people enjoying a certain level of a lifestyle doesn't mean you can, or should, be doing the same."

Ignoring work benefits.

If you're not taking full advantage of your employee benefits, you're leaving money on the table. You should already be contributing to your employer's 401(k) retirement account if it's available and taking full advantage of any company match program, and there are some other overlooked — yet incredibly advantageous — benefits:

Healthcare flexible-spending account: This type of account is a pretax benefit account you can use to cover a variety of healthcare products and services, from acupuncture and physical therapy to vaccines and over-the-counter medicine (note that OTC medicines are only eligible when prescribed by a physician). You can put up to $2,550 of tax-free money into this account in 2015, and save about 30% on healthcare expenses with the tax break, WageWorks reports.

Dependent-care flexible-spending account: If you have young children, dependent-care FSAs are worth considering. This account works similarly to the healthcare FSA, in that you can contribute pretax money, but is specific for dependent-care services, such as preschool, summer camp, daycare, or before- and after-school programs.

Commuter benefits: These are often overlooked, but they can save you over $600 each year, WageWorks tells The Wall Street Journal. The concept is simple: Employees can use pretax money from their paychecks to cover mass-transit passes — including the train, subway, bus, ferry, and parking.

Talk to your human-resources department to understand the scope of what's available to you. These benefits could save you thousands of dollars each year.

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Wasting cash on fees.

Whether it be out of laziness or ignorance, many of us continue to pay fees — for overdrafts, for using out-of-network ATMs, or for missed payments. These seemingly insignificant charges can add up over time.

Not saving for a rainy day.

Unexpected expenses — the wedding gift you forgot to buy, the unlucky parking ticket, or the emergency flight home you had to book — have a way of surfacing when you least expect them. No matter how hard you try to avoid them, they'll continue sneaking up at the most inopportune times.

These savings are separate from your emergency fund. It's easy to ignore the possibility of your car breaking down, a medical emergency, or losing your job, but these are all scenarios that could quickly become expensive realities. Not setting aside money in an emergency fund could ultimately land you in debt if an emergency does arise.

Avoiding the subject of money with your significant other.

They're not fun or easy conversations to have, but discussing your personal finances, spending patterns, and financial plan with your partner is crucial — and the earlier the conversations happen, the better.

If you live together, "Spell out who's responsible for paying which bills," writes David Bach in his book "Smart Couples Finish Rich." "You shouldn't assume that both you and your partner are somehow automatically on the same page when it comes to the question of how you are going to organize your finances and who is going to be responsible for what. If you haven't already done so, the two of you need to sit down together and specifically work all this out. The alternative is chaos and potentially major strife."

Dedicate 2016 to making smart, quality purchases you can afford — even if the sticker price makes you cringe at first glance — and investing in things that have value. They don't have to be big purchases, either. There are several costly everyday items that eventually pay for themselves.

Assuming you'll have more money in the future.

While optimism is a good quality to have, too much optimism can be dangerous, especially when it comes to money. People tend to assume they'll be making significantly more money later on, which they use to justify overspending in the present moment.

The rule of thumb should be to always live below your means. As Michael Egan, a certified financial planner and partner at Egan, Berger, and Weiner, tells Business Insider: "Savings first should be your mentality: Save for retirement first, and spend with whatever is left over. What people typically do is the opposite of that, thinking, 'I've got to buy this, this, and this, and whatever's left, I'll save.' Pay your future first, and make sure your present is secure."